Bearing assembly with built-in absolute encoder

ABSTRACT

The bearing assembly includes a rolling bearing including a rotatable raceway member and a stationary raceway member. This bearing assembly also includes a to-be-detected member carried by the raceway member and having a magnetic characteristic which cyclically varies in a direction circumferentially of the to-be-detected element and has a cycle matching with one complete rotation of the raceway member, a magnetic sensor unit carried by the raceway member in face-to-face relation with the to-be-detected member, and a magnetic detecting circuit to supply an electric power to the unit and to process an output signal from the unit to provide an output to an external circuit. The magnetic characteristic of the to-be-detected member represents a substantially sinusoidal waveform rising and falling more steeply than a sinusoidal waveform at a zero-crossing point.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a bearing assembly having an absoluteencoder built therein, which has a capability of detecting the absoluteangle of rotation and can be employed in various equipments such asarticulates of a robot or articulated manipulator.

2. Description of the Prior Art

In view of the easiness to assemble compact, a rolling bearing assemblyhaving a rotation sensor built therein is currently placed in themarket, an example of which is shown in FIG. 11 of the accompanyingdrawings. The rolling bearing assembly 51 in FIG. 11 includes an innerrace 52, which is a rotatable raceway member, an outer race 53, which isa stationary raceway member and encloses the inner race 52 with acylindrical bearing space defined therebetween, and a circumferentialrow of rolling elements 54 rollingly retained by a roller retainer 55and interposed between the inner and outer races 52 and 53.

An annular magnetic encoder 56 is secured to one end of the inner race52 for rotation together therewith and is employed in the form of, forexample, an annular rubber magnet having a plurality of oppositemagnetic N and S poles alternating with each other in a directioncircumferentially thereof. Cooperable with the annular magnetic encoder56 is a magnetic sensor 57, which is in the form of, for example, a Hallelement and secured to a corresponding end of the outer race 53 inface-to-face relation with the magnetic encoder 56. The magnetic sensor57 is resin molded or encapsulated in a resinous casing 58, which is inturn fixedly mounted on the outer race 53 by means of a metallic casing59.

In the rolling bearing assembly 51 of the above structure, the magneticsensor 57 detects alternating change in magnetic poles of the magneticencoder 56, as shown in FIG. 12, during rotation of the inner race 52and subsequently outputs a train of incremental pulses, as shown in FIG.13, which is descriptive of the number of revolutions, or the rotationalspeed, of the inner race 52.

It has, however, been found that with the rotation sensor of thestructure described above, although the incremental pulse signaldescriptive of the rotational speed of the inner race 52 is obtained,the rotation sensor is unable to provide the absolute angle of rotationof the inner race 52, unless the power-on initialization of the rotationsensor is carried out before counting of the pulses starts.

In order to alleviate the above discussed inconvenience, the JapaneseLaid-open Patent Publication No. 2004-4028, published Jan. 8, 2004, forexample, discloses, as shown in FIG. 14, a bearing assembly with abuilt-in absolute encoder, in which a radial type to-be-detected element61, mounted on an inner race, has a magnetic characteristic varying in asubstantially sinusoidal waveform having a cycle matching with onecomplete rotation of the inner race. A magnetic sensor unit 60 fordetecting change in magnetism of the to-be-detected element 61 iscomprised of two magnetic sensors 60A and 60B arranged at respectivelocations radially outwardly of the to-be-detected element 61 and spaceda predetermined angular distance from each other in a directioncircumferentially of the to-be-detected element 61.

According to the above structure, since the magnetic characteristic ofthe to-be-detected element 61 is so designed as to vary with each cyclematching with one complete rotation of the inner race, the magneticsensor unit 60 can easily output a signal indicative of the absoluteangle of rotation. Also, when an output indicative of the differencebetween respective outputs from the two magnetic sensors 60A and 60B issubjected to rectangular pulse shaping, a rectangular signal of a cyclematching with one complete rotation of the inner race can be obtained asan origin signal, i.e., a signal indicative of the original angularposition of the inner race.

Alternatively, a sinusoidal output generated from one of the twomagnetic sensors 60A and 60B is compared with a center voltageintermediate of the amplitude of such sinusoidal output to provide arectangular signal, which can be used as the origin signal.

However, the absolute encoder disclosed in the above discussed patentpublication is unable to provide the origin signal of a high accuracysince it tends to be adversely affected by variation in threshold valuebetween the magnetic sensors 60A and 60B and/or decrease in magnetism ofthe to-be-detected element 61 under the influence of temperatures. Assuch, where the repeatability of the origin signal is strictly required,it is necessary to employ, in addition to a combination of theto-be-detected element 61 and its cooperating magnetic sensor unit 60associated with the detection of the absolute angle of rotation, anadditional combination of a to-be-detected element and an additionalmagnetic sensor unit associated with the detection of the originalposition, so that an origin signal can be generated.

In order to alleviate the foregoing problems and inconveniences, thepreviously discussed patent publication also discloses an absoluteencoder of an alternative structure, in which as shown in FIG. 15, acombination of a to-be-detected element 67 and a magnetic sensor unit 68for the detection of the absolute angle of rotation and a combination ofa to-be-detected element 87 and a magnetic sensor unit 88 for thedetection of the original position are employed.

Referring to FIG. 15, the inner race 52 carries not only the firstto-be-detected element 67 associated with the detection of the absoluteangle of rotation, but also the second to-be-detected element 87associated with the detection of the original position and, on the otherhand, the outer race 53 carries not only the first magnetic sensor unit68 cooperable with the to-be-detected element 67, but also the secondmagnetic sensor unit 88 cooperable with the to-be-detected element 87.The first to-be-detected element 67 for the detection of the absoluteangle of rotation is of a radial type, in which as shown in FIG. 16A ina transverse sectional representation, the pattern of magnetizationvaries in a substantially sinusoidal waveform with a cycle matching withone complete rotation of the inner race 52. The first magnetic sensorunit 68 for the detection of the absolute angle of rotation is comprisedof two magnetic sensors 68A and 68B arranged at respective locationsradially outwardly of the to-be-detected element 67 and spaced apredetermined angular distance, for example, 90° from each other in adirection circumferentially of the to-be-detected element 67. Usingrespective outputs from the two magnetic sensors 68A and 68B, it ispossible to determine the quadrant and, hence, the absolute angle ofrotation of the inner race 52 can be indicated.

On the other hand, the second to-be-detected element 87 for thedetection of the original position is also of a radial type, in which asshown in FIG. 16B in a transverse sectional representation, the patternof magnetization is such that a pair of magnetic N and S poles iscreated in a direction circumferentially thereof or, alternatively, asingle pole, for example, S pole is created in a directioncircumferentially thereof. The second magnetic sensor unit 88 for thedetection of the original position is comprised of a single magneticsensor of a latch type or a switch type capable of generating an outputsignal corresponding to the magnetic flux density. As the inner race 52rotates, the second magnetic sensor unit 88 provides the origin signalin terms of change in magnetism of the pair of N and S poles of thesecond to-be-detected element 87.

It has, however, been found that if the distance D between the firstto-be-detected element 67 and the second to-be-detected element 87, aswell as the distance D′ between the first magnetic sensor unit 68 andthe second magnetic sensor unit 88, is small as shown in FIG. 17A,respective leakage fluxes from the first and second to-be-detectedelements 67 and 87 will interfere with each other. Where the secondto-be-detected element 87 is magnetized to have a single S pole, themagnetic characteristic of the first to-be-detected element 67 in adirection radially thereof will be such as shown in FIG. 17B and themagnetic characteristic of the second to-be-detected element 87 in adirection radially thereof will be such as shown in FIG. 17C.

Under these conditions, a considerable error tends to occur in theabsolute angle of rotation represented by the output signals from themagnetic sensors 68A and 68B forming respective parts of the firstmagnetic sensor unit 68. Also, the output from the second magneticsensor unit 88 associated with the detection of the original position islatched (or switched) in the vicinity of a region where the magneticflux of the to-be-detected element 87 decreases to zero, not in thevicinity of the single S pole where the magnetic flux of theto-be-detected element 87 varies considerably and, therefore, theaccuracy of the origin detection signal tends to be lowered.

SUMMARY OF THE INVENTION

In view of the foregoing, it is an object of the present invention toprovide a bearing assembly having an absolute encoder built therein,which is capable of detecting the absolute angle of rotation and alsocapable of providing a highly accurate origin signal, i.e., a highlyaccurate signal indicative of the original position, with no need to useany additional to-be-detected element separate from that for thedetection of the absolute angle of rotation.

It is another object of the present invention to provide the bearingassembly having the absolute encoder of the type referred to above,which is effective to accurately detect the absolute angle of rotationand the original position.

In order to accomplish the foregoing objects of the present invention,the present invention in accordance with one aspect thereof provides abearing assembly having an absolute encoder built therein, whichincludes a rolling bearing including a rotatable raceway member, astationary raceway member enclosing the rotatable raceway member with acylindrical bearing space defined between it and the rotatable racewaymember, and a circumferential row of rolling elements rollingly housedwithin the cylindrical bearing space and interposed between therotatable and stationary raceway members. This bearing assembly alsoincludes a to-be-detected element carried by the rotatable racewaymember and having a magnetic characteristic which cyclically varies in adirection circumferentially thereof and has a cycle, relative to themagnetic sensor unit, matching with one complete rotation of therotatable raceway member, a magnetic sensor unit carried by thestationary raceway member in face-to-face relation with theto-be-detected element, and a magnetic detecting circuit for supplyingan electric power to the magnetic sensor unit and for processing anoutput signal from the magnetic sensor unit to provide an output to anexternal circuit. In this bearing assembly, the magnetic characteristicof the to-be detected element represents a substantially sinusoidalwaveform rising and falling more steeply than a sinusoidal waveform at azero-crossing point. The magnetic detecting circuit outputs a detectionsignal indicative of the absolute angle of the rotatable raceway memberand an origin signal indicative of an original position of the rotatablemember.

According to this aspect of the present invention, since the magneticcharacteristic of the to-be detected element varies cyclically with eachcycle matching with one complete rotation of the rotatable racewaymember, it is possible to detect the absolute angle of rotation of therotatable raceway member. Also, since the magnetic characteristic of theto-be-detected element represents a substantially sinusoidal waveformrising and falling more steeply than a sinusoidal waveform at azero-crossing point, it is possible to generate a highly accurate originsignal by the utilization of the output signal from the magnetic sensorunit. Thus, the absolute angle can be detected and, with no need to useany extra to-be-detected element separate from that for the detection ofthe absolute angle, a highly accurate origin signal can be obtained.

It is, however, to be noted that the magnetic detecting circuit may beof a type capable of outputting only the origin signal by processing theoutput signal from the magnetic sensor unit and having no function ofoutputting the detection signal indicative of the absolute angle.

In the practice of the above described aspect of the present invention,the magnetic sensor unit may include a plurality of magnetic sensors, inwhich case at least one of those magnetic sensors may be used as anorigin signal generator for generating the origin signal indicative ofthe original position of the rotatable raceway member.

The use of the plural magnetic sensors makes it easy to detect theabsolute angle of the rotatable raceway member by detecting theto-be-detected element which varies cyclically with each cycle matchingwith one complete rotation of the rotatable raceway member. For theorigin signal generator, either one of the magnetic sensors used for thedetection of the absolute angle or one of the other magnetic sensors maybe employed. If one of the magnetic sensors for the detection of theabsolute angle is used for the detection of the original position, thenumber of the magnetic sensors to be used can advantageously be reduced,resulting in simplification of the structure. On the other hand, if adedicated magnetic sensor is used for the detection of the originalposition, a magnetic sensor having a characteristic suitable for thedetection of the original position can be employed and, therefore, afurther highly accurate origin signal can be obtained.

Preferably, at least one of the magnetic sensors, which is used as theorigin signal generator, may be a Hall IC of a latch type or a switchtype. In this case, the output from the magnetic sensor forming theorigin signal generator is in the form of a rectangular shaped originsignal.

Also preferably, the origin signal may be used for a detection of adirection in which the rotatable raceway member is returned to theoriginal position. By way of example, if the bearing assembly with theabsolute encoder built therein in accordance with the present inventionis incorporated in a device such as a manipulator arm device, in whichthe angle of rotation of a manipulator arm is limited, the direction inwhich the rotatable raceway member, that is, the manipulator arm isreturned to the original position can be readily ascertained. Thus,where the bearing assembly with the absolute encoder built thereinaccording to the present invention is used to rotatably support amovable component, such as the manipulator arm of the manipulator armdevice, of which the angle of rotation is limited, the origin signal maybe utilized in a control unit which drives the movable component, sothat at the time the electric power is turned on, the direction ofrotation of the movable component back to the original position can bedetected.

The present invention in accordance with another aspect thereof alsoprovides a bearing assembly having an absolute encoder built therein,which includes a rolling bearing including a rotatable raceway member, astationary raceway member enclosing the rotatable raceway member with acylindrical bearing space defined between it and the rotatable racewaymember, and a circumferential row of rolling elements rollingly housedwithin the cylindrical bearing space and interposed between therotatable and stationary raceway members. This bearing assembly alsoincludes a first to-be-detected element for a detection of an absoluteangle, which is carried by the rotatable raceway member and has amagnetic characteristic cyclically varying in a directioncircumferentially of the first to-be-detected element, a first magneticsensor unit for the detection of the absolute angle, which is carried bythe stationary raceway member in face-to-face relation with the firstto-be-detected element, a second to-be-detected element for a detectionof an original position, which is carried by the rotatable racewaymember at a location separate from the first to-be-detected element andhas a magnetic characteristic of a single pole or a single pair of N andS poles, a second magnetic sensor unit for the detection of the originalposition, which is carried by the stationary raceway member inface-to-face relation with the second to-be-detected element, and amagnetic detecting circuit for supplying an electric power to the firstand second magnetic sensor units and for processing respective outputsignals from the first and second magnetic sensor units to generate toan external circuit a signal indicative of the absolute angle ofrotation of the rotatable raceway member and an origin signal indicativeof the original position of the rotatable raceway member during onecomplete rotation of the rotatable raceway member relative to thestationary raceway member.

In this bearing assembly according to the second aspect of the presentinvention, a combination of the first to-be-detected element and thefirst magnetic sensor unit for the detection of the absolute angle and acombination of the second to-be-detected element and the second magneticsensor unit for the detection of the original position are so positionedas to be immune from leakage fluxes emanating from the first and secondto-be-detected elements, respectively. In other words, the first andsecond to-be-detected elements and the first and second magnetic sensorunits are properly positioned relative to each other so that the leakageflux emanating from the first to-be-detected element for the detectionof the absolute angle will not substantially affect the sensitivity ofthe second magnetic sensor unit for the detection of the originalposition and the leakage flux emanating from the second to-be-detectedelement for the detection of the original position also will notsubstantially affect the sensitivity of the first magnetic sensor unitfor the detection of the absolute angle.

According to the second aspect of the present invention, the use of thecombination of the first to-be-detected element and the first magneticsensor unit for the detection of the absolute angle and the combinationof the second to-be-detected element and the second magnetic sensor unitfor the detection of the original position is effective to obtain theabsolute angle of rotation of the rotatable raceway member and alsoeffective to provide the origin signal indicative of the originalposition of the rotatable raceway member during one rotation of therotatable raceway member, with no need to perform any power-oninitialization. In particular, since in this aspect of the presentinvention the combination of the first to-be-detected element and thefirst magnetic sensor unit for the detection of the absolute angle andthe combination of the second to-be-detected element and the secondmagnetic sensor unit for the detection of the original position are sopositioned that they will not be adversely affected by the leakagefluxes emanating from the first and second to-be-detected elements, thedetection signal outputted from each of the first and second magneticsensor units is free from any adverse influence by the leakage fluxes,and, therefore, the absolute angle of rotation and the origin signal canbe outputted with high accuracy.

In the practice of the second aspect of the present invention, the firstto-be-detected element may have a magnetic characteristic cyclicallyvarying with each cycle matching with one complete rotation of therotatable raceway member and the first and second to-be-detectedelements are preferably disposed on a common cylindrical surface butspaced axially from each other.

According to this feature, since the magnetic characteristic of thefirst to-be-detected element varies cyclically with each cycle matchingwith one complete rotation of the rotatable raceway member, a simplifiedcalculating process can be carried out in the magnetic detecting circuitto provide the absolute angle of rotation. Also, since the first andsecond to-be-detected elements are preferably disposed on the commoncylindrical surface, the first and second to-be-detected elements can bedisposed with a simplified structure.

Also, in one embodiment, the first to-be-detected element and the secondto-be-detected element may be axially spaced a distance of 1 mm or more.If the axial distance between the first and second to-be-detectedelements is 1 mm or more, it is possible to protect the respectivesensitivities of the first and second magnetic sensor units from beingadversely affected by the leakage fluxes emanating from the first andsecond to-be-detected elements, respectively.

Preferably, in one embodiment, the first to-be-detected element may havea magnetic characteristic cyclically varying with each cycle matchingwith one complete rotation of the rotatable raceway member and thesecond to-be-detected element may be disposed with its direction of amagnetic field offset 90° from the direction of the magnetic field ofthe first to-be-detected element.

In this structural feature, since the magnetic characteristic of thefirst to-be-detected element for the detection of the absolute anglevaries cyclically with each cycle matching with one complete rotation ofthe rotatable raceway member, it is possible to easily obtain theabsolute angle of rotation. Also, since the second to-be-detectedelement is disposed with the direction of its magnetic field offset 90°from that of the first to-be-detected element, as compared with the casein which the first and second to-be-detected elements are arranged inthe same direction, it is possible to protect the combination of thefirst to-be-detected element and the first magnetic sensor unit and thecombination of the second to-be-detected element and the second magneticsensor unit from being adversely affected by the leakage fluxesemanating from the first and second to-be-detected elements, even thoughthe distances, over which the first and second to-be-detected elementsare spaced, are small.

Also, where the second to-be-detected element is disposed with itsdirection of a magnetic field offset 90° from the direction of themagnetic field of the first to-be-to-be-detected element, the first andsecond to-be-detected elements are preferably spaced an axial distanceof 0.5 mm or more and a radial distance of 0.5 mm or more from eachother.

Where the second to-be-detected element is disposed with the directionof its magnetic field offset 90° from the direction of the magneticfield of the first to-be-to-be-detected element, selection of the axialdistance and the radial distance between the first to-be-detectedelement and the second to-be-detected element to be 0.5 mm or more iseffective to protect the detection of the absolute angle and theoriginal position from being adversely affected by the leakage fluxesemanating from the first and second to-be-detected elements.

BRIEF DESCRIPTION OF THE DRAWINGS

In any event, the present invention will become more clearly understoodfrom the following description of preferred embodiments thereof, whentaken in conjunction with the accompanying drawings. However, theembodiments and the drawings are given only for the purpose ofillustration and explanation, and are not to be taken as limiting thescope of the present invention in any way whatsoever, which scope is tobe determined by the appended claims. In the accompanying drawings, likereference numerals are used to denote like parts throughout the severalviews, and:

FIG. 1A is a fragmentary longitudinal sectional view of a bearingassembly with a built-in absolute encoder according to a first preferredembodiment of the present invention;

FIG. 1B is a cross-sectional view taken along the line I-I in FIG. 1A;

FIG. 2 is a diagram showing the pattern of magnetization in ato-be-detected element employed in the bearing assembly;

FIG. 3 is a chart showing waveforms of a origin signal generated by amagnetic sensor unit employed in the bearing assembly;

FIG. 4A is a top plan view of a manipulator arm device employing thebearing assembly of the present invention;

FIG. 4B is a cross-sectional view taken along the line IV-IV in FIG. 4A;

FIG. 5 is an explanatory diagram showing the relation between theposition of a second arm of the manipulator arm device and the originsignal;

FIG. 6 is a fragmentary longitudinal sectional view of a bearingassembly with a built-in absolute encoder according to a secondpreferred embodiment of the present invention;

FIG. 7A is a cross-sectional view taken along the line IIa-IIa in FIG.6;

FIG. 7B is a cross-sectional view taken along the line IIb-IIb in FIG.6;

FIG. 8 is a chart showing waveforms of an output generated by a magneticsensor unit for the detection of the absolute angle of rotation, whichis employed in the bearing assembly according to the second embodimentof the present invention;

FIG. 9A is a fragmentary longitudinal sectional view, showing on anenlarged scale a portion of the bearing assembly of FIG. 6;

FIG. 9B is a diagram showing the pattern of magnetization of a firstto-be-detected element for the detection of the absolute angle ofrotation, which is employed in the bearing assembly of FIG. 6;

FIG. 9C is a diagram showing the pattern of magnetization of a secondto-be-detected element for the detection of the original position, whichis employed in the bearing assembly of FIG. 6;

FIG. 10A is a fragmentary longitudinal sectional view of a bearingassembly with a built-in absolute encoder according to a third preferredembodiment of the present invention;

FIG. 10B is a fragmentary longitudinal sectional view, on an enlargedscale, showing a portion of the bearing assembly shown in FIG. 10A;

FIG. 11 is a longitudinal sectional view of the conventional bearingassembly with the built-in absolute encoder;

FIG. 12 is an explanatory diagram showing the relation in positionbetween a magnetic sensor unit and a magnetic encoder both employed inthe conventional bearing assembly shown in FIG. 11;

FIG. 13 is a diagram showing the waveform of a detection signalgenerated by the magnetic sensor unit employed in the conventionalbearing assembly shown in FIG. 11;

FIG. 14 is an explanatory diagram showing the relation between ato-be-detected element and a magnetic sensor unit both employed in thesuggested bearing assembly shown in FIG. 11;

FIG. 15 is a fragmentary longitudinal sectional view of anothersuggested bearing assembly with a built-in absolute encoder;

FIG. 16A is a cross-sectional view taken along the line Xa-Xa in FIG.15;

FIG. 16B is a cross-sectional view taken along the line Xb-Xb in FIG.15;

FIG. 17A is a fragmentary longitudinal sectional view, showing on anenlarged scale a portion of the bearing assembly shown in FIG. 15;

FIG. 17B is a diagram showing the pattern of magnetization of ato-be-detected element for the detection of the absolute angle ofrotation; and

FIG. 17C is a diagram showing the pattern of magnetization of ato-be-detected element for the detection of the original position.

DETAILED DESCRIPTION OF THE EMBODIMENTS

A first preferred embodiment of the present invention will be describedin detail with particular reference to FIGS. 1A to 5.

As shown in FIG. 1A, a rolling bearing assembly with a built-in absoluteencoder according to a first embodiment of the present inventionincludes a rolling bearing unit 1 made up of a rotatable raceway member2, a stationary raceway member 3 enclosing the rotatable raceway member2 with a cylindrical bearing space defined between it and the rotatableraceway member 2, and a circumferential row of rolling elements 4rollingly retained by a roller retainer 5 and interposed between therotatable and stationary raceway members 2 and 3. The rotatable racewaymember 2 and the stationary raceway member 3 are rotatable relative toeach other. The rolling bearing assembly with the built-in absoluteencoder also includes a to-be-detected element 7 secured to one end ofthe rotatable raceway member 2, a magnetic sensor unit 8 secured to oneend of the stationary raceway member 3 in face-to-face relation with theto-be-detected element 7, and a magnetic detecting circuit 9. Themagnetic detecting circuit 9 is preferably in the form of a printedcircuit board having electric and/or electronic component parts mountedon one surface thereof, but may be in the form of an IC chip or anyother circuit chip.

The rolling bearing unit 1 may be in the form of a deep groove ballbearing having inner and outer races, which define the rotatable andstationary raceway members 2 and 3, respectively. the rotatable racewaymember 2 has an outer peripheral surface formed with at least oneraceway groove 2 a and the stationary raceway member 3 has an innerperipheral surface formed with a mating raceway groove 3 a, and therolling elements 4 rollingly retained by the roller retainer 5 andinterposed between the rotatable and stationary raceway members 2 and 3are in part received in the raceway groove 2 a and in part in theraceway groove 3 a. One of opposite annular open ends of the cylindricalbearing space delimited between the rotatable and stationary racewaymembers 2 and 3, which is remote from the to-be-detected element 7 andthe magnetic sensor unit 8, is sealed by a sealing member 6.

The to-be-detected element 7 is of a radial type and is in the form ofan annular component having a magnetic characteristic, relative to themagnetic sensor unit 8, cyclically varying in a directioncircumferentially thereof. The magnetic characteristic of the element 7varies cyclically relative to the magnetic sensor unit 8, with a cyclematching with one complete rotation of the rotatable raceway member 2.

More specifically, as shown in FIG. 1B, the to-be-detected element 7 ismade up of an annular backing metal 12 and a magnetic member 13 providedon an outer peripheral surface of the backing metal 12 and having N andS poles so magnetized as to alternate with each other in a directioncircumferentially thereof. The magnetic member 13 has a pattern ofmagnetization represented by a substantially sinusoidal waveform havinga cycle matching with one complete rotation of the rotatable racewaymember 2. In particular, the substantially sinusoidal waveformrepresenting the magnetic characteristic of the magnetic member 13 is sochosen that, as represented by any one of substantially sinusoidalwaveforms B, C and D shown by broken lines in FIG. 2, steeper rise andfall than those of the sinusoidal waveform A can be exhibited in thevicinity of a zero-crossing point, at which the magnetic flux attainszero. It is to be noted that the substantially sinusoidal pattern ofmagnetization exhibits the rise and fall, as discussed above, which aresteeper than those of the sinusoidal pattern of magnetization, in whichthe surface areas each bound by a half-cycle portion of the sinusoidalwaveform and the coordinate axis representing the zero value remain thesame.

The to-be-detected element 7 is fixedly mounted on the rotatable racewaymember 2 through the backing metal 12. The magnetic member 13 is in theform of, for example, a rubber magnet bonded by vulcanization to thebacking metal 12, but may be in the form of a plastic magnet or asintered magnet, in which case the use of the backing metal 12 is notalways essential and may be dispensed with.

As shown in FIG. 1A, the magnetic sensor unit 8 is made up of first andsecond magnetic sensors 8A and 8B each operable to generate an outputsignal corresponding to the magnetic flux density. As shown in FIG. 1B,the first and second magnetic sensors 8A and 8B, each being in the formof an analog sensor, are so arranged as to be spaced a predeterminedangular distance, for example, 90° from each other in a circumferentialdirection. In practice, the first and second magnetic sensors 8A and 8Bare surface mounted on a printed circuit board forming a part of themagnetic detecting circuit 9 and are, after having been inserted into aresinous casing 10 together with the printed circuit board of themagnetic detecting circuit 9, encapsulated in a resin molding. Theresinous casing 10 so resin molded is fixed to the stationary racewaymember 3 by means of a metallic casing 11 to secure the first and secondmagnetic sensors 8A and 8B and the magnetic detecting circuit 9 to thestationary raceway member 3.

The magnetic detecting circuit 9 is operable to supply an electric powerto the magnetic sensor unit 8 and to process an output signal from themagnetic sensor unit 8 before it generates an output signal to anexternal circuit. This magnetic detecting circuit 9 processes the outputsignal from the magnetic sensor unit 8 to generate an output indicativeof the absolute angle of rotation of the rotatable raceway member 2 andan output indicative of the original position of the rotatable racewaymember 2. The magnetic detecting circuit 9 may be disposed at a locationexternal to the rolling bearing unit 1.

In the bearing assembly with the built-in absolute encoder of thestructure described above, since the first and second magnetic sensors8A and 8B are spaced in 90° phase offset relation to each other and theto-be-detected element 7 with magnetization of a substantiallysinusoidal pattern is provided in which each cycle of the substantiallysinusoidal waveform matches with one complete rotation of the rotatableraceway member 2, a quadrant determination is possible from therespective outputs of the first and second magnetic sensors 8A and 8Band, therefore, the absolute angle of rotation of the rotatable racewaymember 2 can be measured.

Also, since the to-be-detected element 7 has such a magneticcharacteristic that the substantially sinusoidal waveform rises andfalls steeply in the vicinity of the zero-crossing point, not only caninfluences brought about by variation in threshold value between thefirst and second magnetic sensors 8A and 8B and/or demagnetization ofthe magnet by temperature be minimized, but also a highly accurateorigin signal indicative of the original position of the rotatableraceway member 2 can be generated using the respective outputs from thefirst and second magnetic sensors 8A and 8B. For this reason, thebearing assembly of the present invention can be conveniently used in anapplication where the repeatability of the origin signal is strictlyrequired.

In such case, generation of the origin signal may be accomplished eitherby shaping a signal indicative of the difference between the respectiveoutputs from the first and second magnetic sensors 8A and 8B to providea rectangular signal of a cycle matching with one complete rotation ofthe rotatable raceway member 2 to obtain the origin signal indicative ofthe original position of the rotatable raceway member 2, or by comparingthe output from one of the magnetic sensors 8A and 8B with a centervoltage corresponding to a value intermediate of the amplitude of thesinusoidal waveform to provide the rectangular signal to thereby obtainthe origin signal.

By way of example, if the magnetic characteristic of the to-be-detectedelement 7 is such that the magnetic flux varies gradually in thevicinity of the zero crossing point, at which the magnetic flux attainszero, in a manner similar to the sinusoidal waveform A shown by thesolid line in FIG. 2, the accuracy of detection of the absolute anglecan be improved, but the origin signal generated by the above describedprocess of detecting the original position will lack the repeatabilityof detection of the original position. However, where the magneticcharacteristic of the to-be-detected element 7 is such that the magneticflux steeply changes in the vicinity of the zero crossing point asrepresented by any one of the substantially sinusoidal waveforms B, Cand D shown in FIG. 2, the bearing assembly can be conveniently used inan application where the repeatability of the origin signal is strictlyrequired although the accuracy of detection of the absolute angle may bemore or less lowered.

Also, as a modification of the illustrated embodiment, a third magneticsensor 8C, which forms a means for generating the origin signal, may beemployed in addition to the first and second magnetic sensors 8A and 8B.In such case, the third magnetic sensor 8C should be positioned at alocation radially outwardly of the to-be-detected element 7 in coaxialrelation therewith and in an out-of-phase relation with the first andsecond magnetic sensors 8A and 8B as shown by the phantom line in FIG.1B. The third magnetic sensor 8C is in the form of a digital outputsensor of a single sided or half wave magnetic responsive type (that is,a switch type) or of an alternating or full wave magnetic responsivetype (that is, a latch type). For the third magnetic sensor 8C, a HallIC, for example, can be employed.

The origin signal generated when the third magnetic sensor 8C isemployed in the form of the digital output sensor of the alternatingmagnetic responsive type is shown in FIG. 3. As can readily be seen fromFIG. 3, the origin signal can be obtained. Accordingly, in such case,the absolute angle can be obtained relying on the respective outputsfrom the first and second magnetic sensors 8A and 8B, which provide asubstantially sinusoidal waveform signal and, on the other hand, theoriginal position can be obtained relying on the output from the thirdmagnetic sensor 8C, which provides a rectangular waveform signal.

Alternatively, as a second modification of the foregoing embodiment, oneof the first and second magnetic sensors 8A and 8B may be dispensedwith, in which case the substantially sinusoidal output from one of thefirst and second magnetic sensors 8A and 8B and the substantiallyrectangular output from the third magnetic sensor 8C are utilized togenerate the angle detection signal indicative of the absolute angle. Insuch case, the absolute angle and the original position can be obtainedfrom those two signals.

Again alternatively, as a third modification of the foregoingembodiment, the magnetic detecting circuit 8 may be of a circuit design,in which no means for calculating the absolute angle is employed, butonly the output indicative of the original position can be generated. Insuch case, only one of the first to third magnetic sensors 8A to 8C isemployed.

FIGS. 4A and 4B illustrate plan and sectional views of a manipulator armdevice 15 utilizing the bearing assembly with the built-in absoluteencoder according to the embodiment and modifications. The manipulatorarm device 15 includes a first arm 16 fixedly coupled with the outerrace 3 of the bearing assembly 14 and a second arm 17 fixedly coupledwith the inner race of the bearing assembly 14. In this application, therespective angles of rotation of the first and second arms 16 and 17 arelimited by mechanical interference. By way of example, when the secondarm 17 is driven by an actuator (not shown) as shown by the broken linein FIG. 4A, although the absolute position of the second arm 17 can bedetermined by the bearing assembly 14 at the time the device 15 ispowered on, it is necessary, where the position of the arm is desired tobe detected highly accurately, to perform an initialization to returnthe second arm 17 back to the original position by the utilization of ahighly accurate origin detecting function possessed by the bearingassembly 14. During this initialization, in order to avoid themechanical interference, it is necessary to rotate the second arm 17 ina direction, in which the second arm 17 is rotated during the returnmovement, that is, in a direction towards the original position of thesecond arm 17. In such case, a means to determine which direction thesecond arm 17 should be driven relative to the original position isnecessary.

The manner in which the direction of rotation of the arm back to theoriginal position is detected using only the origin signal will now bedescribed. Let it be assumed that referring to FIG. 4A, at the originalposition a of the second arm 17 the bearing assembly 14 performs adetection of the original position. When the second arm 17 is driven,the relation between the origin signal generated by the magnetic sensor8C or the origin signal generated by processing the detection signaloutputted from either one of the magnetic sensors 8A and 8B and theposition of the second arm 17 is such as shown in FIG. 5. In otherwords, assuming, for example, that the second arm 17 has been driven tothe position b shown in FIG. 4A, the origin signal is held in a highlevel state and, therefore, it can be ascertained that the direction inwhich the second arm 17 has to be driven in order for it to return tothe original position a from the position b is a counterclockwisedirection. On the other hand, if the second arm 17 has been driven tothe position c, the origin signal is held in a low level state and,therefore, it can be ascertained that the direction in which the secondarm 17 has to be driven in order for it to return to the originalposition a from the position c is a clockwise direction.

In this embodiment, the direction towards the original position may bedetected from a detected value of the absolute position of the arm, butwith the means utilizing the origin signal shown in FIGS. 4A to 5, thedirection of rotation of the second arm 17 back to the original positioncan be detected with no need to detect the absolute angle. Because ofthis, the magnetic detecting circuit may be of a type from which acalculating circuit or the like for calculating the absolute angle iseliminated.

Referring now to FIG. 6, there is shown the bearing assembly with thebuilt-in absolute encoder according to a second preferred embodiment ofthe present invention. The bearing assembly includes a rolling bearingunit 1 made up of a rotatable raceway member 2, a stationary racewaymember 3 enclosing the rotatable raceway member 2 with a cylindricalbearing space defined between it and the rotatable raceway member 2, anda circumferential row of rolling elements 4 rollingly retained by aroller retainer 5 and interposed between the rotatable and stationaryraceway members 2 and 3. The bearing assembly also includes a firstto-be-detected element 7 for the detection of the absolute angle, asecond to-be-detected element 27 for the detection of the originalposition, a first magnetic sensor unit 8 for the detection of theabsolute angle, a second magnetic sensor unit 28 for the detection ofthe original position, and a magnetic detecting circuit 9. The first andsecond to-be-detected elements 7 and 27 are axially juxtaposed relativeto each other and are secured to one end of the rotatable raceway member2, whereas the first and second magnetic sensor units 8 and 28 areaxially juxtaposed relative to each other and are secured to one end ofthe stationary raceway member 3 in face-to-face relation with the firstand second to-be-detected elements 7 and 27.

The rolling bearing unit 1 may be in the form of a deep groove ballbearing having inner and outer races, which define the rotatable andstationary raceway members 2 and 3, respectively. The rotatable racewaymember 2 has an outer peripheral surface formed with at least oneraceway groove 2 a and the outer race or stationary raceway member 3 hasan inner peripheral surface formed with a mating raceway groove 3 a. Therolling elements 4 are in part received in the raceway groove 2 a and inpart in the raceway groove 3 a. One of opposite annular open ends of thecylindrical bearing space delimited between the rotatable and stationaryraceway members 2 and 3, which is remote from the first and secondto-be-detected elements 7 and 27 and the magnetic sensors 8 and 28, issealed by a sealing member 6.

The first to-be-detected element 7 is of a radial type and is in theform of an annular component having a magnetic characteristic cyclicallyvarying in a direction circumferentially thereof relative to themagnetic sensor 8. The magnetic characteristic varies cyclically with acycle matching with one complete rotation of the rotatable racewaymember 2.

More specifically, as shown in FIG. 7A, the to-be-detected element 7 ismade up of an annular backing metal 12 and a magnetic member 13 providedon an outer peripheral surface of the backing metal 12 and having N andS poles so magnetized as to alternate with each other in a directioncircumferentially thereof. The magnetic member 13 has a pattern ofmagnetization represented by a substantially sinusoidal waveform witheach cycle matching with one complete rotation of the rotatable racewaymember 2. The to-be-detected element 7 is fixedly mounted on therotatable raceway member 2 through the backing metal 12. The magneticmember 13 is in the form of, for example, a rubber magnet bonded byvulcanization to the backing metal 12, but may be in the form of aplastic magnet or a sintered magnet.

As shown in FIG. 7A, the magnetic sensor unit 8 for the detection of theabsolute angle is made up of first and second magnetic sensors 8A and 8Beach operable to generate an output signal corresponding to the magneticflux density. The first and second magnetic sensors 8A and 8B, eachbeing in the form of an analog sensor, are so arranged as to be spaced apredetermined angular distance, for example, 90° from each other in acircumferential direction. The first and second magnetic sensors 8A and8B are mounted on a circuit board having the magnetic detecting circuit9.

FIG. 8 illustrates a chart showing the waveforms of detection signalsoutputted from the first and second magnetic sensors 8A and 8B as therotatable raceway member 2 rotates. The use of the first and secondmagnetic sensors 8A and 8B displaced in phase relative to each other iseffective to accomplish the quadrant determination and, therefore, theabsolute angle of rotation of the rotatable raceway member 2 can bemeasured from the respective outputs of the first and second magneticsensors 8A and 8B. Such a signal processing for the detection of theabsolute angle is carried out by an absolute angle calculating circuit(not shown) provided in the magnetic detecting circuit 9. It is,however, to be noted that the signal processing may be carried out by anexternal circuit.

Referring again to FIG. 6, the second to-be-detected element 27 is madeup of the annular backing metal 12 and a second magnetic member 22provided on the outer peripheral surface of the backing metal 12. Thesecond magnetic member 22 is separate from the first magnetic member 13of the first to-be-detected element 7 for the detection of the absoluteangle, but positioned on the outer peripheral surface of the backingmetal 12 or a cylindrical surface common to the first magnetic member 13and axially next to the first magnetic member 13. This second magneticmember 22 may be of a radial type and is in the form of a sheet-likecomponent having a magnetic characteristic varying in a directioncircumferentially thereof relative to the second magnetic sensor unit 28for the detection of the original position. The magnetic characteristicof the second magnetic member 22 of the second to-be-detected element 27varies circumferentially with at least one polarity in thecircumferential direction. In the illustrated embodiment, as shown inFIG. 7B, the second to-be-detected element 27 has a pattern ofmagnetization defined by a pair of N and S poles. Where themagnetization pattern is defined by a single pole, a single pole magnetis utilized. This magnetic member 22 may be employed in the form of arubber magnet and is bonded to the backing metal 12.

As shown in FIG. 9A on an enlarged scale, in the second preferredembodiment, the axial distance D1 between the magnetic member 13 of thefirst to-be-detected element 7 and the magnetic member 22 of the secondto-be-detected element 27 is so chosen as to prevent the leakage fluxes,leaking from the first and second to-be-detected elements 7 and 27, fromadversely affecting the corresponding magnetic sensor units 8 and 28.The distance D1 is, for example, 1 mm or more.

As shown in FIG. 6, the second magnetic sensor unit 28 for the detectionof the original position is made up of one or more magnetic sensorscapable of generating an output signal corresponding to the magneticflux density. In the illustrated embodiment, only one magnetic sensor isemployed for the second magnetic sensor unit 28. A magnetic sensorforming the magnetic sensor unit 28 for the detection of the originalposition is in the form of a digital sensor of a single sided or halfwave magnetic responsive type or of an alternating or full wave magneticresponsive type. The magnetic sensor unit 28 is mounted on the samecircuit board of the magnetic detecting circuit 9, on which the magneticsensors 8A and 8B of the magnetic sensor unit 8 for the detection of theabsolute angle are mounted and is, after having been inserted into theresinous casing 10 together with the circuit board, resin molded. Incorrespondence with the respective positions of the to-be-detectedelements 7 and 27, the magnetic sensor units 8 and 28 are axiallyjuxtaposed to each other.

As shown in FIG. 9A, the axial distance D2 between the first magneticsensor unit 8 and the second magnetic sensor unit 28 is also so chosenas to avoid interference between the leakage flux emanating from thefirst to-be-detected element 7 and that from the second to-be-detectedelement 27. The distance D2 is, for example, 1 mm or more. As shown inFIG. 6, with the resinous casing 10 fixed to the stationary racewaymember 3 through the metallic casing 11, the first and second magneticsensors forming respective parts of the magnetic sensor unit 8 for thedetection of the absolute angle, the magnetic sensor unit 28 for thedetection of the original position and the circuit board of the magneticdetecting circuit 9 are thus carried by the stationary raceway member 3.It is to be noted that the magnetic detecting circuit 9 is utilized as acircuit operable to supply an electric power to the first and secondmagnetic sensors 8A and 8B and the magnetic sensor unit 28 and also toprocess the respective output signals from the first and second magneticsensor units 8 and 28 before it provides an output signal. This magneticdetecting circuit 9 may be in the form of a printed circuit board havingelectric and/or electronic component parts mounted on one surfacethereof, but may be in the form of an IC chip or any other circuit chip,or may be disposed outside the bearing assembly.

According to the foregoing embodiment shown in and described withparticular reference to FIGS. 6 to 7B, since the magnetic characteristic(for example, the magnetic strength in the illustrated embodiment)cyclically varies with a cycle matching with one complete rotation ofthe rotatable raceway member 2 to which the to-be-detected element 7 isfixed, the absolute angle of rotation of the stationary raceway member 2can be ascertained with no need to perform the power-on initialization.Also, since a combination of the to-be-detected element 27 and themagnetic sensor unit 28 is utilized to generate the origin signalindicative of the original position of the rotatable raceway member 2during one complete rotation of the rotatable raceway member 2, theoriginal position of the rotatable raceway member 2 can be detectedassuredly.

In this embodiment, the to-be-detected element 7 for the detection ofthe absolute angle has such a magnetic characteristic as to varysubstantially sinusoidally and the magnetic sensor unit 8 is made up ofthe first and second magnetic sensors 8A and 8B arranged in phasedifference. Also, an output signal indicative of the phase differencebetween the first and second magnetic sensors 8A and 8B is utilized bythe absolute angle calculating circuit of the magnetic detecting circuit9. Accordingly, the absolute angle of rotation of the rotatable racewaymember 2 can be accurately ascertained.

Also, in this embodiment, since the to-be-detected element 27 for thedetection of the original position is disposed on the same cylindricalsurface of the backing metal 12, on which the to-be-detected element 7for the detection of the absolute angle is disposed, but spaced adistance axially from the first to-be-detected element 7, the first andsecond to-be-detected elements 7 and 27 can be arranged with asimplified structure.

In particular, in this embodiment, the axial distance D1 between thefirst and second to-be-detected elements 7 and 27 and the axial distanceD2 between the first and second magnetic sensor units 8 and 28 are eachchosen to be 1 mm or more and, as shown in FIG. 9A on an enlarged scale,leakage fluxes emanating from the first and second to-be-detectedelements 7 and 27 will not interfere with each other between acombination of the first to-be-detected element 7 and the first magneticsensor 8 and a combination of the second to-be-detected element 27 andthe second magnetic sensor 28. Accordingly, the magnetic characteristicof the first to-be-detected element 7 will represent a sinusoidalwaveform with no deformation as shown in FIG. 9B and, on the other hand,the magnetic characteristic of the second to-be-detected element 27 willbe such as shown in FIG. 9C, wherein the magnetic characteristic only ata location where the magnetic pole pair is disposed varies locally and,therefore, the absolute angle of rotation and the original position ofthe rotatable raceway member 2 can be ascertained further accurately.

FIG. 10 illustrates a third preferred embodiment of the presentinvention. The bearing assembly of the third embodiment is similar tothat of the first embodiment, except that the second magnetic member 22for the detection of the original position is of an axial type and,correspondingly, the second magnetic sensor 28 for the detection of theoriginal position is so disposed as to confront axially with the secondmagnetic member 22. In such case, as shown in FIG. 10B on an enlargedscale, the first magnetic member 13 forming a part of the firstto-be-detected element 7 and the magnetic member 22 forming a part ofthe second to-be-detected element 27 are spaced an axial distance D3 anda radial distance D3 from each other. The axial distance D3 and theradial distance D4 are each chosen to be, for example, 0.5 mm or more sothat leakage fluxes emanating from the first and second to-be-detectedelements 7 and 27 will not interfere with each other between thecombination of the first to-be-detected element 7 and the first magneticsensor unit 8 and the combination of the second to-be-detected element27 and the second magnetic sensor unit 28. Accordingly, the absoluteangle of rotation and the original position of the rotatable racewaymember 2 can be ascertained accurately.

In this third embodiment, since the second to-be-detected element 27 forthe detection of the original position is so disposed that the directionof the magnetic field emanating from the second to-be-detected element27 can be offset 90° relative to that emanating from the firstto-be-detected element 7 for the detection of the absolute angle, it ispossible to protect both the combination of the first to-be-detectedelement 7 and the first magnetic sensor unit 8 and the combination ofthe second to-be-detected element 27 and the second magnetic sensor unit28 from being adversely affected by the leakage fluxes emanating fromthe first and second to-be-detected elements 7 and 27, even though thedistances D3 and D4, over which the first and second to-be-detectedelements 7 and 27 are spaced, are small.

Also, considering that the axial distance D3 and the radial distance D4are chosen to be 0.5 mm or more, it is possible to ensure that thecombination of the first to-be-detected element 7 and the first magneticsensor unit 8 and the combination of the second to-be-detected element27 and the second magnetic sensor 28 can be protected from beingadversely affected by the leakage fluxes emanating from the first andsecond to-be-detected elements 7 and 27.

Although the present invention has been fully described in connectionwith the preferred embodiments thereof with reference to theaccompanying drawings which are used only for the purpose ofillustration, those skilled in the art will readily conceive numerouschanges and modifications within the framework of obviousness upon thereading of the specification herein presented of the present invention.Accordingly, such changes and modifications are, unless they depart fromthe scope of the present invention as delivered from the claims annexedhereto, to be construed as included therein.

1. A bearing assembly having an absolute encoder built therein,comprising: a rolling bearing including a rotatable raceway member, astationary raceway member enclosing the rotatable raceway member with acylindrical bearing space defined between it and the rotatable racewaymember, and a circumferential row of rolling elements rollingly housedwithin the cylindrical bearing space and interposed between therotatable and stationary raceway members; a to-be-detected membercarried by the rotatable raceway member and having a magneticcharacteristic cyclically varying in a direction circumferentiallythereof, the magnetic characteristic, relative to the magnetic sensorunit, having a cycle matching with one complete rotation of therotatable raceway member; a magnetic sensor unit carried by thestationary raceway member in face-to-face relation with theto-be-detected member; and a magnetic detecting circuit to supply anelectric power to the magnetic sensor unit and to process an outputsignal from the magnetic sensor unit to provide an output to an externalcircuit, wherein the to-be-detected member is magnetized to have itsmagnetic flux partially saturated on both a north-pole side and asouth-pole side, whereby the to-be-detected member shows the waveform ofthe magnetic flux having the slope steeper around a zero-crossing pointthan that of a complete sinusoidal waveform which does not show suchsaturation and having a cycle matching with one complete rotation of therotatable raceway member.
 2. The bearing assembly having the absoluteencoder built therein as claimed in claim 1, wherein the magnetic sensorunit includes a plurality of magnetic sensors, one of which sensors isused as an origin signal generator for generating an origin signalindicative of an original position of the rotatable raceway member. 3.The bearing assembly having the absolute encoder built therein asclaimed in claim 2, wherein one of the magnetic sensors, which is usedas the origin signal generator, is a Hall IC of a latch type or a switchtype.
 4. The bearing assembly having the absolute encoder built thereinas claimed in claim 2, wherein the origin signal is utilized for adetection of a direction in which the rotatable raceway member isreturned to the original position.